Converters are widely used for various power conversion and control applications, including converting sources of direct current (DC) electrical energy into alternating current (AC) power supplies. In particular, current-fed resonant converters are widely used for various DC and AC powers supplies, due to their efficient and compact features.
A successful application of current-fed resonant converters is in Inductively Coupled Power Transfer (ICPT) systems. Other applications include induction heating.
In these and other applications of resonant converters, undesirable frequency shifts can occur, which are subject to load or circuit parameter variations. These can result in significant reduction of system power transfer capacity.
These problems can be overcome, but this requires complex design which is expensive to implement, such as third generation (G3) power supplies using complicated LC converting networks.
One example of the difficulties created by frequency shifts is resonant converter power supplies in ICPT systems. These systems (which are also known as contactless power supplies) are known to have significant advantages in applications such as the materials handling, lighting and transportation industries. There are many applications in both high and low power systems in which use of these power supplies is particularly advantageous.
ICPT systems have a primary conductive path supplied with current from a power supply which is usually a resonant converter such as a current-fed resonant converter. One or more secondary devices (which may be referred to as pick-ups) are provided adjacent to, but electrically isolated from, the primary conductive path. The pick-ups have a pick up coil in which a voltage is induced by the magnetic field associated with the primary path, and supply a load such as an electric motor, a light, or a sensor, for example. The pick-up coil is tuned using a tuning capacitor to increase power transfer to the pick-up.
The load that each pick-up supply will typically fluctuate or vary, and this load change is reflected through the mutual inductive coupling back to the supply, which affects the supply frequency. This frequency drift in turn has an adverse effect on the pick-ups because the tuning capacitor for each pick-up tunes the pick-up coil to the resonant frequency of the converter. As the converter resonant frequency drifts away, the power transferred to the pick-ups decreases, so the system becomes inefficient. Further information about the construction and design of current-fed push/pull resonant converters, particularly as they apply to ICPT applications, can be found with reference to the specification of U.S. Pat. No. 5,450,305 assigned to Auckland UniServices Limited. Further information on ICPT systems, power supplies and pick-ups for such systems can be found with reference to the specification of U.S. Pat. No. 5,293,308 also assigned to Auckland UniServices Limited.
One approach that has been used in an attempt to maintain the frequency of a converter circuit at or near resonance in response to variations in load has been to provide a plurality of capacitors in the resonant circuit which may be switched in or out of the circuit. This approach has been proposed in recently published United States patent application US2003/0210106. Switching a plurality of individual capacitors into or out of the resonant circuit means that the circuit frequency can only be controlled in a stepwise fashion. This is particularly disadvantageous with high Q systems since many separate capacitors are required, adding to cost and complexity. It also means that load variations have to be limited to make the system function effectively.